Various devices and arrangements have been attempted to provide a continuous readout of the drill bit depth as drilling operations are conducted to drilling as well bore or as pipe or other objects are raised and lowered in a well bore. Generally speaking, all of such devices have presented various problems, some of which are due to the fact that the drill string on which the drill bit is connected is generally formed by joints of pipe, and the drill string is supported for up and down movement in a drilling mast at the earth's surface. The drill line is anchored at one end, called the dead line anchor, generally adjacent a leg of the drilling mast. The drill line extends from the anchor upwardly to a crown block formed of a plurality of rotatable sheaves supported on top of the upper end of the drilling mast. The drill line is reeved about the sheaves in the crown block and extends from one of the sheaves down to a sheave in what is termed a traveling block and then back up to a sheave in the crown block, etc., until the desired number of sheaves in the crown block and traveling block have the cable received thereon, and the drill line then extends from the crown block downwardly to a powered rotatable drum in what is normally termed the drawworks. The traveling block is provided with suitable means for removably connecting with the drill string to suspend it in the well bore, but enabling the traveling block to be disconnected from the drill string when necessary to conduct other operations.
During drilling or the other operations, measuring or displaying the depth of the drill bit, pipe, or other objects is complicated in that uninterrupted and continuous lowering of the drill string, pipe or other objects, during drilling, hoisting and other operations is not practical. For example, there are devices available on the market that measure the total depth of a sand line, or wireline, in a well bore which is normally lowered into the well bore in substantially a continuous and uninterrupted manner. It can be appreciated that when the lowering operation is interrupted and more particularly when the measuring arrangement for determining the total depth of the drill bit on a drill string, pipe or other objects is operably associated with and responsive to the up and down movement of the drill line, the mesuring arrangement must be stopped and started at a time to correspond with the up and down movement of the drill line or cable. This is particularly true since the traveling block, when it is disconnected from the drill pipe or drill string, is used a lot of times without the drill pipe ever moving or without the bit moving up or down in the well bore.